In a medium saucepan, add potatoes and garlic - add enough water to cover and season with salt. Bring to a boil - reduce heat and simmer until the potatoes are tender, about 15 minutes. Reserve 1/4 cup of the cooking water, then drain well. Place potatoes back into the saucepan and return over low heat. Gently stir potatoes, allowing any excess liquid to evaporate away. Add butter and reserved cooking water - mash potatoes with a potato masher.

Meanwhile, in a medium skillet, heat 1 tablespoon oil over medium-high heat. Season chicken with salt and fresh ground black pepper - add chicken to the skillet and cook until done, about 4 minutes per side. Remove to a plate and let cool slightly - shred chicken into bite-sized pieces.

In the same skillet, add remaining teaspoon of oil to the skillet. Add onion, carrots and thyme and cook, stirring, until the onions have softened, about 4 minutes. Stir in 1 1/2 cups chicken broth and bring to a boil - scrape up any brown bits from the bottom of the skillet. Lower heat down to medium and stir in craping up any browned bits. Lower the heat to medium and add broccoli and peas - cook for 1 minute.

In a medium bowl, whisk together remaining 1/2 cup broth and cornstarch - stir into the vegetable mixture. Bring to a boil and cook until thickened. Stir in the chicken and mustard - season with salt and fresh ground black pepper.

Divide mashed potatoes between 4 serving dishes and make a well in the center of each. In each well, place an even amount of the chicken and vegetable mixture to serve.